The present invention relates to apparatus and method for drilling horizontal underground boreholes, in particular horizontal underground boreholes requiring a close tolerance on-grade slope or horizontal segment.
The present invention is directed to a method for drilling a close tolerance on-grade subsurface borehole using a dual member drill string and a downhole tool assembly. The dual member drill string comprises an outer member and an inner member disposed within the outer member and rotatable independent of the outer member. The method comprises rotating the downhole tool solely by rotating the inner member of the drill string and simultaneously advancing the downhole tool assembly to bore a substantially straight segment of the borehole. The method further comprises changing direction of the borehole by advancing the downhole tool assembly without rotation.
In another aspect the invention is directed to a downhole tool assembly for a dual member drill string, the dual member drill string comprising an outer member and an inner member disposed within the outer member. The downhole tool assembly comprises a bearing housing assembly, a directional boring tool, and a drive member. The bearing housing assembly is connectable to a downhole end of the outer member of the drill string and is characterized by an outer wall defining an interior bearing chamber with a straight central axis. The drive member has a front portion, a body, and a rear portion. The front portion is adapted to be operatively connected in torque transmitting engagement to the directional boring tool. The body is supported within the interior bearing chamber. The rear portion is operably connectable in torque transmitting engagement with a downhole end of the inner member of the drill string.
In yet another aspect, the present invention is directed to a horizontal directional drilling machine comprising a frame, a dual member drill string, a dual-rotary drive system, and a downhole tool assembly. The drill string comprises an outer member and an inner member disposed generally coaxially within the outer member, the outer member and inner members having first respective ends and second respective ends. The dual-rotary drive system is attachable to the frame and operatively connectable to the first respective ends of the dual-member drill string. The dual-rotary drive system is adapted to rotate and advance the drill string. The downhole tool assembly comprises a bearing housing assembly, a directional boring tool, and a drive member. The bearing housing assembly is connectable to the second end of the outer member and is characterized by an outer wall defining an interior bearing chamber with a straight central axis. The drive member is characterized by a front portion, a body, and a rear portion. The front portion is operably connectable in torque transmitting engagement to the second end of the inner member. The body is supported within the interior bearing chamber. The rear portion is adapted to be operatively connected in torque transmitting engagement to the directional boring tool.